The present invention relates to reflow soldering of articles, and more particularly to the mounting of electrical circuit components to substrates by reflow soldering.
Electronic components and substrates (typically printed circuit boards) frequently are electrically and mechanically bonded by solder reflow operations. Solder reflow involves applying a solid solder alloy in paste form to the substrate, placing electrical components against the substrate in their intended locations, heating the printed circuit board and components to a temperature above the melting point of the solder alloy, then cooling these components to solidify and establish the electrical and mechanical connections. In what is known as SMT (surface mount technology), printed circuit boards are conveyed in a sequence for application of the solder alloy in paste form, automated placement of circuit components onto the printed circuit board, and a reflow operation that is "in-line", i.e. performed upon the circuit boards and components as they move in sequence.
Known SMT reflow systems are available from Research Inc. of Minneapolis, Minn.; Vitronics Corporation of New Market, N.H.; Manncorp of Huntingdon Valley, Pa.; and Conceptronic of Portsmouth, N.H. In these systems, endless belt conveyors or rails are employed to carry PCB/component assemblies serially and at a constant rate (linear velocity) through a tunnel. The tunnel is selectively heated to provide several different heat zones. The necessary heating is accomplished by natural convection, forced convection, infrared heating or a combination of these approaches. As it progresses through the tunnel, each PCB/component assembly is subject to a "profile" that is a function of line speed and conditions within the various zones. While temperature is the primary variant among zones, other conditions can be varied, e.g. by introducing nitrogen or another inert gas into one or more selected zones.
In general, batch processing equipment is well known as an alternative to in-line processing. In the fabrication of electronic assemblies, batch ovens are commonly used at temperatures below solder reflow temperatures, for moisture removal, curing of adhesives, and environmental testing. Batch ovens can be sealed, which facilitates more precise control of the environment inside the oven. However, because of the required manual handling and slow throughput rates as compared to in-line systems, batch ovens are not considered practical for performing solder reflow operations in a manufacturing environment.
At the same time, the very characteristics that facilitate automation and high throughput for in-line systems, also give rise to several problems. One is the difficulty in precisely controlling the heated tunnel environment. The tunnel must be open at its entrance and exit ends to allow continuity in product movement. It is difficult to control the environment within the tunnel, particularly within zones near the entrance or the exit. Because some interaction between adjacent zones is inevitable, all zones are more difficult to precisely control. More particularly, there are temperature gradients between adjacent zones, and any nitrogen introduced into one of the zones tends to flow into the adjacent zones. Certain desired steps, e.g. drawing a vacuum, are impossible in these tunnels since they must be open at both ends to allow continuous product movement. Thus, in-line systems impose definite limits upon the precision and degree of control over the reflow profile.
Continuous component movement during reflow also imposes minimum size requirements upon in-line equipment. Heated tunnels typically are in the range of 4-12 feet in length, with corresponding total equipment lengths in the range of about 12-20 feet. For a given reflow profile, an increase in throughput requires a corresponding increase in the length of each heat zone and, of course, the tunnel itself. Greater length increases the cost of a system and the floor space required to accommodate it. The moving endless belt conveyor imparts vibration to components during and immediately after reflow, which can diminish the quality and consistency of soldered connections.
Therefore, it is an object of the present invention to provide an automated reflow soldering system that affords more precise control of the reflow profile.
Another object is to provide a reflow soldering enclosure that facilitates more rapid changes in environment immediately surrounding the components undergoing reflow soldering.
A further object is to provide a reflow soldering system in which several enclosures, each performing a reflow soldering operation upon a stationary component assembly, are merged into a fully automated in-line fabrication process.
Yet another object is to provide a reflow soldering system conveniently adjustable to achieve a wide range of throughput rates.